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5.1: Capillaries

How Do Oxygen and Nutrients Get From Blood to Cells?

Capillaries are the body's smallest blood vessels. Blood flowing in capillaries nourishes body cells with nutrients and oxygen and receives waste materials such as carbon dioxide. In this section you will find out more about capillaries-their structure, how they function, and what they do for your body.

Did You Know? Capillaries are tiny! They are only about the width of a hair. You have about capillaries in your body.

There are capillaries in the lungs and in every organ and tissue in the body. They act as the body's exchange vessels. Gases, food nutrients, water, and wastes pass back and forth between body cells and the bloodstream across the thin walls of capillaries.

Let's follow nutrients from the digestive tract to see how the capillaries and arterioles work together. First, remember that the arterioles control the blood that enters the capillaries. Now suppose you eat a large meal. When you begin to digest your food, the arterioles that are connected to capillaries in your intestine open. Blood flows to the intestines and takes up nutrients from your food. When you begin to exercise, the arterioles to your muscles open so the blood can carry nutrients to your muscle cells. When the arterioles going to the muscles open, some arterioles to your intestine close down. Blood moves from your intestines to your muscles carrying the needed nutrients.

The walls of capillaries are made up of very thin, flat cells. The capillary walls keep large structures such as red blood cells inside the capillary. Other substances such as oxygen, carbon dioxide, glucose (sugar), and water can pass through.

Did You Know? Blood coming out of the heart travels at per second. By the time it reaches the capillaries it travels about of a centimeter ( of an inch) per second.

Figure 4.1 A web-like structure of capillaries, called a capillary bed, receives blood from an arteriole. The blood drains into a small vein called a venule (VEEN-yool.) The venule returns blood to larger veins and finally back to the heart.

Think about what happens to the water you drink. Where does it go after it gets to your stomach? Water molecules squeeze through the walls of your intestines and enter nearby capillaries. The water molecules that squeeze through the intestine wall mix with water molecules already in your blood plasma. Together the water molecules circulate in your bloodstream-pushed along by the pumping action of your heart. Eventually these water molecules pass through the walls of other capillaries and enter the spaces between body cells. In this way, water moves everywhere in your body. So the water you drink can go to your brain, your toes, your liver, and everywhere else in your body. Water moves everywhere because it passes through capillary walls.

Now let's see how blood moves from an arteriole to the capillaries that supply body tissues. Tissues are the sheets of cells that make up body organs. Every body organ has networks of capillaries that supply its tissues with blood. Look at Figure 4.1. This drawing illustrates how blood moves through capillaries.

Follow the flow of blood in Figure 4.1 starting at the arteriole. Remember the high blood pressure of the arteries drops in the arterioles. The pressure drops even more when the blood leaves the arteriole and branches into the many capillaries. The blood pressure is low as the blood moves through the capillaries. Blood moves slowly from the arteriole through the twisted web of capillaries. At each turn the blood flow may slow down or even turn around and go backwards for a second before going forward again. As blood reaches the end of the capillary bed, the capillaries start joining together. The blood starts flowing a little faster when it enters a venule (little vein). Then the blood flows into larger veins and finally back to the heart.

Figure 4.2 The thin walls of a capillary bed and the slow movement of blood through it allow nutrients, gases, and wastes to be exchanged between the blood and the tissues.

Did You Know? It takes to for blood to pass through a capillary. Capillaries hold about of the body's blood at any given time.

Did You Know? Sometimes red blood cells, which carry oxygen for delivery to the body's cells, have to go single file into the capillaries. Some capillaries are so tiny that the red blood cells have to bend and squeeze to get through.

Transport of Nutrients: Exploring Diffusion

Answer the following questions in writing.

What do you think would happen if you filled a small beaker with tap water and added one drop of food coloring?

What would happen if you filled a small beaker with tap water and added a cube of sugar?

How is diffusion affected by temperature?

What happens when someone opens a container of perfume or uses a spray bottle of room deodorizer?

Design your own activity to explore one of these questions. Then answer this question. How is the diffusion described here like the diffusion of nutrients and wastes in your cells?

Summary

Let's summarize what happens in capillaries. Blood moves very slowly through capillaries. As the blood moves through a capillary, nutrients, oxygen, and food leave the blood and enter the body cells. The blood also picks up wastes and carbon dioxide. The blood in capillaries works like a mail carrier. The mail carrier goes to each house on the route delivering mail and picking up letters to be mailed. The blood flows through the capillaries picking up and delivering materials as it passes each cell.

Capillaries go everywhere in the body. No cell in your body is more than two cells away from a capillary. Capillaries are exchange vessels. Gases (oxygen and carbon dioxide), nutrients, and wastes pass in both directions across capillary walls. Blood flow in capillaries is pushed by the pumping of the heart. Since water gets though cell membranes easily, the circulating blood moves water through every cell of the body.

Activity 4-1: Making a Capillary Bed Model

Introduction

You learned that capillaries have an important role in bringing nutrients and oxygen to your cells and in carrying away waste materials and carbon dioxide. Use your creativity to build a model of a capillary bed to help you learn more about the role of capillaries.

small artery (arteriole) carrying oxygen-rich blood with nutrients to the capillaries

small vein (venule) taking oxygen-poor blood with wastes away from the capillaries

Step 2 Construct and identify the parts of your model. Use arrows to show the direction of blood flow.

Step 3 Identify in your model where oxygen and nutrients leave the capillaries cells and where wastes and carbon dioxide leave the cells and enter the capillaries.

Step 4 Record your names and date on your model.

Red Blood cells

Pretend you are a red blood cell.

Describe your journey as you pass through a capillary bed. How are you different leaving the capillary bed than when you entered the capillary bed?

How do materials move from the blood in the capillaries into your body cells? The exchange of oxygen and carbon dioxide and the exchange of nutrients and waste materials happen due to a process called diffusion (dih-FYOO-shun). Diffusion is the random movement of molecules from a region of higher concentration to a region of lower concentration. For example, a cell uses oxygen and produces carbon dioxide. The oxygen-rich blood from the heart has a lot of oxygen and much less carbon dioxide. The oxygen diffuses from the blood, where there is a greater concentration, to the cell, where there is a lower concentration. The carbon dioxide diffuses from the cell, where there is a greater concentration, to the blood, where there is a lower concentration. Given enough time, diffusion leads to an even distribution of molecules.

Review Questions

How are capillaries different from arterioles?

Why do you have capillaries?

Describe and explain the process by which substances enter and leave capillaries.